Storage media for computers and other types of electronic devices generally come in two types: volatile memory and non-volatile memory. Volatile memory loses its contents when power is no longer being supplied to the memory, whereas non-volatile memory maintains its contents even when power is not being supplied to the memory. The most common type of volatile memory is dynamic random-access memory (DRAM), which is most commonly available as and implemented as an integrated circuit (IC). The term data storage medium is used herein in a broad sense, and encompasses IC memory, as well as other types of data storage media.
By comparison, non-volatile memory has perhaps more commonly been available as and implemented as magnetic and optical media, including hard disk drives, floppy disks, compact disc read-only memories (CD-ROM's), CD re-writable (CD-RW) discs, and digital versatile discs (DVD's), among others. Historically, non-volatile memory implemented as an IC was primarily available as ROM that was not re-recordable, such as hard-wired ROM and programmable ROM (PROM). More recently, IC non-volatile memory has become available as various types of flash memory, which is more technically known as electrically erasable PROM (EEPROM).
It is a general aim for the computer industry to increase the storage density of the storage media being used by computers. Every new technology, however, should offer long-term perspectives in order to give room for continued improvements within the new technology. This is due to the fact that with every fundamental change of storage technology, the computer industry has to undertake remarkable investments in order to adapt existing production machines or to replace existing machines by new ones for any technical purpose involved with said new technology. Thus, the consequence for further development of storage systems is that any new technique with better storage area density should have a long-term potential for further scaling, desirably down to the nanometer scale.
The only available tool known today that is simple and yet provides these very long term perspectives is a nanometer probe tip. Such tips are used in every atomic force microscope (AFM) and scanning tunneling microscope (STM) for imaging and structuring down to the atomic scale. The simple tip is a very reliable tool that concentrates on one functionality: the ultimate local confinement of interaction.
In recent years, AFM thermo-mechanical recording in polymer storage media has undergone extensive modifications mainly with respect to the integration of sensors and heaters designed to enhance simplicity and to increase data rate and storage density. Using heated cantilevers, thermo-mechanical recording at 400 Gb/in2 storage density and data rates of a few Mb/s for reading and 100 kb/s for writing have been demonstrated.
Such prior art thermo-mechanical writing is a combination of applying a local force by the cantilever/tip to a polymer layer and softening it by local heating. By applying sufficient heat an indentation can be formed into the storage medium for writing a bit which can be read back with the same tip, by the fact that the lever is bent when it is moved into the indentation and the electrical resistance of a sensing circuit is changed therewith.
While writing data or bits, the heat transfer from the tip to the polymer through the small contact area is initially very poor and improves as the contact area increases. This means the tip must be heated to a relatively high temperature to initiate the melting process. Once melting has commenced, the tip is pressed into the polymer, which increases the heat transfer to the polymer, increases the volume of melted polymer, and hence increases the bit size. After melting has started and the contact area has increased, the heating power available for generating the indentations increases by at least ten times to become 2% or more of the total heating power (depending on the design). In order to provide a complete data storage method, a read process must be proposed that can provide adequate Signal-to-Noise Ratio (SNR) at an acceptable data rate.
A conventional method for reading data depends on the modulation of the gap between a warm (non-writing) cantilever and the data storage medium. The gap modulation is produced by the tip following medium topography introduced by the writing process just described. The described gap modulation generates a synchronous modulation in the cantilever temperature through a variation in thermal flux between cantilever and medium. The temperature coefficient of resistivity of the heater translates this temperature variation into a resistance variation, which is sensed by appropriate electronics as the output signal. However, the bandwidth or data rate for this read method is fundamentally limited by the thermal time constant of the heater/cantilever.
Accordingly, what is needed is a method and system that is capable of reading these bits at a higher data rate with a similar or improved SNR. The method and system should be simple and capable of being easily adapted to existing technology. The present invention addresses these needs.